For many years now fixed bed gasifiers have been employed to produce syngas for running internal combustion engines and other means of power production. Fixed bed gasifiers act as a whole with each process tied into what is occurring with the other processes. Thus, it is difficult to separate the respective steps so that each process can be isolated from the other processes making control more difficult.
Conventional fixed bed gasifiers rely on gravity to move the solid fuel through the zones of gasification. Gravity feed is not conducive to handling solids because of the tendency of solid fuels toward bridging, tunneling, and clogging if the solid fuel is not the correct size, shape, density or moisture content. The applicant's prior patent refers to a solids pump such as a bulk solids pump manufactured and distributed by K-Tron Company of Pitman, N.J., USA. The pump is known for its ability to provide precise volumetric feeding of free flowing bulk materials, e.g. pellets and granules. The bulk solids pump feeder has a rotating spool that creates a product lock-up zone conveying the material smoothly from storage hopper located above a consolidation zone to an outlet. True, linear mass flow of particles is achieved. This principle is referred to herein as lock-up transport.
Bulk solids pumps employ one or more specially designed rotating spools which create curved passages. Incoming material is captured in the curved passage and rotated to discharge. While within the curved passage the material ‘locks-up’ and acts as a solid mass. Approaching discharge, inter-particle forces relax and discharge occurs by cascade at the material's natural angle of repose, or by the material being forced out by a barrier placed inside the curved passage which directs the locked-up material out of the curved passage.
The bulk solids pump principle is best explained as consisting of four sequential zones: (1) consolidation, (2) rotation, (3) relaxation and (4) discharge wherein in the first zone material enters the pump and consolidates as particles settle and come into contact with one another and the sides of the curved passage and at the end of the first zone the material is fully constrained by the curved passage, and inter-particle and surface contact forces produce the lock-up condition and wherein in the second zone the material rotates as a solid mass and as the material moves through the second zone to the third zone the material is approaching an unconstrained discharge where inter-particle forces fall, the material relaxes and regains its natural flowability and discharge of the material occurs in a fourth zone where passage rotation causes the material to cascade from the pump.
Fixed bed gasifiers rely on gravity to move fuel through the process steps of heating, pyrolysis, reduction and gasification. This limits the residence time in each of the processes because the position of the feedstock is not controlled and limits the residence depth because of the fixed contours of the apparatus and therefore fixed depth that a process can have.
The aspect ratios of fixed bed gasifiers are limiting. For example, the throat of an Imbert style gasifier is restricted to certain ratios of nozzle size and spacing to throat opening. Operating outside these constraints leads to incomplete gasification, tunneling and increased tar production and/or dilution of the gas energy density. For example the depth of the reduction zone is entangled with the amount of heat that can be created in the throat of the gasifer, which is limited by the fixed contours of the gasifier. Once this heat is exhausted by the endothermic reactions occurring in the reduction zone no more reduction reactions are favored.
The aspect ratio of an Imbert or downdraft gasifier, that is the ratio of the cross sectional area of the gasifier to its length needs to be larger compared with the apparatus in accordance with the present invention in order to support gravity feed. Because of this it is difficult to transfer waste heat back into the process where it is needed to improve efficiency. This leads to energy waste as the waste heat is dumped to the environment. In addition, Imbert or downdraft gasifiers are batch run machines requiring periodic cleaning and reloading.
The Imbert or downdraft machines require a portion of the feedstock to be burnt to create heat for heating and driving the endothermic processes of reduction and tar cracking. This combustion process introduces extra amounts of N2 into the syngas stream if the oxidant is air which dilutes the energy density of the syngas. The apparatus in accordance with the present invention uses waste heat or syngas combustion to drive the processes and these heat sources can remain external to the syngas quality itself.
Downdraft machines and other machines that rely on gravity for solid fuel feeding can only provide a limited amount of feedstock heating because of the aspect ratio and gravity dependent feedstock solid fuel feed issues presented above.
The applicant's prior patent describes advantageous apparatus and methods for improved delivery of solid fuel to a gasifier. It is now apparent that the prior art does not show various improvements that expand the possible processes that can be completed in such solids pumps More specifically, various structural changes in the pump enable recycling of process heat and or process gases to increase the efficiency of processes including torrefaction, pyrolysis, combustion, reduction and or gasification within the solids pump.
From the above, it is therefore seen that there exists a need in the art to overcome the deficiencies and limitations described herein and above.